Hydrocarbons, such as oil and gas, are commonly obtained from subterranean formations that may be located onshore or offshore. The development of subterranean operations and the processes involved in removing hydrocarbons from a subterranean formation typically include a number of different steps such as, for example, drilling a wellbore from a surface location to a desired target in the reservoir, treating the wellbore to optimize production of hydrocarbons, and performing the necessary steps to produce and process the hydrocarbons from the subterranean formation.
The drilling part of completing a well can present many challenges, especially in those formations, which are difficult to drill, such as highly interbedded formation, hard formations or complicated geological structures. Those formations, which require access through complex angles such as is required with directional drilling can also present many challenges as can those formations having many differing structures throughout their depth.
In some drilling applications, it is necessary to enlarge the wellbore to a greater diameter than the drill bit and/or the pass-through diameter of the previous casing string. This can be required for different reasons, the main one being to reduce the circulating pressure of drilling fluid or cement in the wellbore.
Such an operation is commonly known as reaming. This is often accomplished using a device known as a reamer or underreamer. A reamer is included as part of the BHA and attached above the drill bit assembly. The reamer is a secondary drilling apparatus having cutters, which remain retracted within the BHA until it is desired to drill the enlarged hole above the drill bit assembly. There are many mechanisms used to expand and retract the reamer from the BHA, which are well known within the art.
In some applications, especially those involving formations having inter-beds of different strength or structures that intersect the wellbore at different angles, which vary from region to region, the reamer can cut at a different speed than the drill bit, cutting their respective formations at differing depths per unit of time, faster or slower depending on the rock strength. This change in loading between the two cutting structures causes different levels of compression and tension within the BHA above the bit and below the reamer and also above the reamer. This variation in load can cause the borehole to become spiraled as the orientation of the cutting faces is altered as the compression or tension bends the drill collars between the two cutting structures by varying amounts. Different amounts of wear are also induced on the cutting structures by failing to balance the load causing a greater difference in the rates at which the reamer and bit will drill.
If the spiraling is severe enough it is possible for the BHA to become lodged in the wellbore. Spiraling builds up torque on the stabilizers or other down-hole equipment in contact with the formation. This can adversely affect the drilling operation by reducing the rate of penetration, causing premature wear to the cutting structures, increasing the difficulty of moving cuttings out of the wellbore as it becomes spiraled and potentially causing the BHA to become stuck either through the mechanical creation of ledges or excessive cuttings build up. Thus, there remains a need in the art for minimizing spiraling of the borehole in an effort to prevent the BHA from becoming stuck in the borehole during back reaming and to improve overall drilling performance.